1. Statement of the Technical Field
The inventive arrangements relate generally to methods and apparatus for antennas and feed systems, and more particularly to ring focus antennas and feed systems that can operate in multiple frequency bands.
2. Description of the Related Art
It is often desirable for microwave satellite communication antennas to have the ability to concurrently operate on multiple frequency bands. In those situations where a single coaxial feed for multiple bands is desired, it can be challenging to maintain existing system performance specifications without changing the design of an existing main reflector. Further, space limitations associated with existing designs can severely restrict design options with regard to the form factor of the sub-reflector and feed horns.
U.S. Pat. No. 6,211,834 B1 to Durham et al. (hereinafter Durham), concerns a multi-band shaped ring focus antenna. In Durham, a pair of interchangeable, diversely shaped, close proximity-coupled sub-reflector-feed pairs are used for operation at respectively different spectral frequency bands. Swapping out the subreflector/feed pairs changes the operational band of the antenna. Accordingly, that system does not offer concurrent operation on spectrally offset frequency bands.
U.S. Pat. No. 5,907,309 to Anderson et al. and U.S. Pat. No. 6,323,819 to Ergene each disclose dual band multimode coaxial antenna feed horns. However, one problem with coaxial horns of this type is that there is a substantial tendency of horns for different bands to interact with one another due to their coaxial arrangement. These interactions can be exacerbated by co-locating the horn apertures at a substantially common plane. Still, substantially co-located horn apertures have proven to be necessary in many instances due to the need to maintain a common phase center.
The necessity to maintain a common phase center for each horn typically arises where the horns are used to feed a conventional reflector type antenna system. For example, the combination of a particular main reflector and sub-reflector pair for a ring focus antenna will generally establish a required phase center for the feed horns. Where a coaxial horn feed system is used, each horn in the coaxial feed must satisfy the common phase center location requirements.